Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle or hub. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information, and the discs must be rotated at a constant known speed.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor that drives a hub which supports the discs for rotation. Such motors, which are commonly referred to as spindle motors, have a spindle or sleeve mounted by means of two ball bearing or one or more hydrodynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets causes the spindle or hub to rotate, thereby rotating the disc and passing the information storage tracks underneath the head. This arrangement of stator teeth or laminations, coils and magnets may be mounted either inside the hub or spindle, or lower on the shaft below the hub or spindle in order to make the cross-sectional area occupied by the spindle or rotating sleeve as small as possible.
Currently, disk drive spindle motors are being operated at increasingly higher speeds in order to speed up access times and increase storage capacities. In current spindle motor design, the problem arises from the fact that the magnet, as described above, has a plurality of poles. The inventors have recognized that when a magnet with a plurality of poles is moving at high speed near a piece of metal, then the stray flux emitted by the magnet may interact with that metal and create a drag on the rotating magnet. The faster the speed of the rotation of the magnet past the metal, (such as is found in the flange or base of the housing of a disc drive), the more drag is created. This can create a serious power loss in the disc drive system. Thus the problem presented is to prevent undue distribution of stray magnetic flux and reduce the undesirable power losses. At higher speeds the system power increases due to windage loading, bearing losses, magnetic coupling, etc. It is highly desirable that power dissipation be prevented from increasing as drive performance is incremented. With this requirement, this patent seeks to reduce system losses due to magnetic fringing from the spindle motor magnet. Two components of magnet losses are being addressed here, one is Magnetic Hysteresis and is directly related to magnetic angular velocity. Power hyst=d phi d T and is=2*pi*RPM/60*number of magnetic poles.
Another more significant loss factor is eddy current losses which is an exponential function. Power eddy=(2*pi*RPM/60*number of poles)^2.